Various optical scanning apparatus such as laser barcode scanners are widely used in diverse environments for purposes of object identification, data-entry and the like. The scanning apparatus have been developed to read and decode optical indicia, such as barcode symbols, that are attached, printed or otherwise fixed to the object to be identified. During operation of such apparatus, a focused light beam is produced from a light source such as a visible laser diode (VLD), and repeatedly scanned across the elements of the code symbol. In the case of barcode scanning applications, the elements of the code symbol consists of a series of bar and space elements of varying width. For discrimination purposes, the bars and spaces have different light reflectivity (e.g., the spaces are highly light-reflective while the bars are highly light-absorptive). As the laser beam is scanned across the barcode elements, the bar elements absorb a substantial portion of the laser beam power, whereas the space elements reflect a substantial portion of the laser beam power. As a result of this scanning process, the intensity of the laser beam is modulated in accordance with the information structure encoded within the scanned barcode symbol.
As the laser beam is scanned across the barcode symbol, a portion of the reflected light beam is collected by optics within the scanner. The collected light signal is subsequently focused upon a photodetector within the scanner which, in one example, generates an analog electrical output signal which can be decomposed into a number of signal components, namely: a digital scan data signal having first and second signal levels, corresponding to the bars and spaces within the scanned code symbol; ambient-light noise produced as a result of ambient light collected by the light collection optics of the system; thermal noise produced as a result of thermal activity within the signal detecting and processing circuitry; and “paper” or substrate noise, which may be produced as a result of the microstructure of the substrate in relation to the cross-sectional dimensions of the focused laser scanning beam, or noise related to the barcode printing quality (e.g., barcode edge roughness, unwanted spots, void defects, and/or printing contrast).
The analog scan data signal has positive-going transitions and negative-going transitions which signify transitions between bars and spaces in the scanned barcode symbol. However, a result of such noise components or operating the scanner near the operational limits of the focal zones, the transitions from the first signal level to the second signal level and vice versa are not perfectly sharp, or instantaneous. Consequently, it is sometimes difficult to determine the exact instant that each binary signal level transition occurs in the detected analog scan data signal.
The ability of a scanner to accurately scan an encoded symbol character and accurately produce digital scan data signals representative of a scanned barcode symbol in noisy environments depends on the depth of modulation of the laser scanning beam. The depth of modulation of the laser scanning beam, in turn, depends on several important factors. Among the factors are (i) the ratio of the laser beam cross-sectional dimensions at the scanning plane to the width of the minimal barcode element in the barcode symbol being scanned; (ii) the signal-to-noise ratio (SNR) in the scan data signal processor at the stage where binary level (1-bit) analog to digital (A/D) signal conversion occurs; (iii) the object distance; and (iv) the field of view (FOV) angle.
As a practical matter, it is not possible in most instances to produce analog scan data signals with precisely-defined signal level transitions. Therefore, the analog scan data signal must be further processed to precisely determine the point at which the signal level transitions occur. Various circuits have been developed for carrying out such scan data signal processing operations. Typically, signal processing circuits capable of performing such operations include filters for removing unwanted noise components, and signal thresholding devices for rejecting signal components which do not exceed a predetermined signal level. One drawback to these approaches is that thermal and “paper” (or substrate) noise imparted to the analog scan data input signal tends to generate “false” positive-going and negative-going transitions in the first derivative signal, and may also generate zero-crossings in the second-derivative signal. Consequently, the circuit logic allows “false” first derivative peak signals and second-derivative zero-crossing signals to be passed on, thereby producing erroneous binary signal levels at the output stage of the signal processor. In turn, error-ridden digital data scan data signals are transmitted to the digital scan data signal processor of the barcode scanner for conversion into digital words representative of the length of the binary signal levels in the digital scan data signal. This can result in significant errors during barcode symbol decoding operations, causing objects to be incorrectly identified and/or erroneous data to be entered into a host system.